In the German Patent application DE-OS No. 25 14 988 there has already been shown, using a multiple radiation sources containing X-ray tubes, how to record coded images on several successive films, the coded images consisting in each case of shadow images superimposed on one another to more or lesser degrees. From these coded images it is possible, in a later step, to reconstruct images of layers of the object (see German patent application DE-OS No. 27 19 386 and 27 46 035).
The images of layers of an object (DE-OS 25 14 988) are obtained in principle from shadow images, but the latter are not present in an isolated but in an overlapped form, then because of this overlap additional artefacts are transferred into the reconstructed image in the process of layer representation and thus impair the picture quality. The influence of the artefacts becomes greater with increasing degree of overlap. The artefacts can in fact be removed; but the method requires multichannel decoding devices for decoding the overlapping shadow images recorded in each of the respective planes. By means of separate shadow images in each recording plane, however, it is possible to avoid the above artefacts and thus considerably improve the image quality. In the case of separate images, however, the object size is severely limited unless film formats that are too big are used. Using a beam geometry that is technically feasible (approximately 25 tubes, focus-object distance FOD =1200 mm (object-film distance OFA=500 mm)) and a commercial film format of 40.times.40 cm.sup.2 it is only possible therefore to record objects with a diameter of approximately 50 mm, separately. In objects of this size, however, it is very difficult for the doctor to make an accurate diagnosis because, with this size, orientation towards adjacent, known and larger object details (for example vessels filled with contrast medium), which can be an important aid to a doctor making his diagnosis, is usually impossible because this information is not present in the images or lies at an unfavourable spot, for example at the edge of the image. It is desirable, therefore, that an image first be prepared of a relatively large object area, which will enable the doctor to orientate himself. An additional exposure, however, means a greater radiation load on the patient. The patient has also to be subjected to more contrast medium if this is being used for exposure purposes. The longer examination period that results is also, of course, a disadvantage for both patient and doctor.